1. Field of the Invention
The present invention relates to a thin film luminescent element, and more particularly to an electroluminescent(EL) element emitting white light of excellent tone.
2. Description of the Prior Art
Recently, thin display panels have been developed in place of cathode-ray tube. And the EL elements, liquid crystal luminescent elements and plasma luminescent elements or the like are known as the above described display panel.
The EL elements are superior to the liquid luminescent elements in that the operable temperature range is wide and the response property is excellent. And the EL elements are also superior to the plasma elements in that the operable voltage is comparatively low, and the resistance against vibrations is excellent and a thin film can be formed because the whole element is composed of a solid body.
Under the above circumstances, there have been tried to compose a color display panel of the EL element having the above described advantages. For example, Japanese unexamined Patent publication No. Sho 61-499999 proposes an EL element having a luminescent film for emitting light of a wide wave length range and a filter film through which light of a desired wave length range out of the light emitted from the luminescent film is selectively transmitted.
In the EL element proposed by the above publication, the luminescent film is formed by adding PrF.sub.3 as a luminescent center to ZnS as a basic material. The emission spectrum of the luminescent film extends over a wide wave length range from 460 nm to 800 nm. But the luminous intensity in the wave length range from 460 to 480 nm, which corresponds to that of blue out of primary colors is low and accordingly full color of uniform tone cannot be obtained.
Furthermore, Japanese unexamined patent publication No. Sho 60-286128 discloses a thin film EL element composed of a first luminescent film including no luminescent center and a second luminescent film formed on the first luminescent film and having a luminescent center with a predetermined concentration. The second luminescent film having preferable crystallinity, provides luminescence of high luminous intensity.
In addition, Japanese unexamined Patent publication No. sho 61-122080 discloses another thin film EL element of which the substrate is formed of a thermal conductive metal or ceramics.